Building on their brilliant performance against Arsenal, Nottingham Forest humiliated FA Cup holders Leicester with a 4-1 defeat.
Chelsea were without head coach Thomas Tuchel after he tested positive for COVID-19 a few hours before kick-off.
Anthony Elanga’s miss saw Boro celebrate a famous penalty kicks shootout victory at Old Trafford.
In 30 games with Tuchel in charge, Chelsea won 19 games in all competitions, winning Champions League and placing fourth in the Premier League.
The two sides meet again on Tuesday in a crucial English Premier League (EPL) date at Stamford Bridge.
Jamie Vardy, who wasted the best chance of the opening period, got free down the left and crossed for in-form Iheanacho.
Chelsea manager Thomas Tuchel has now defeated Pep Guardiola for the first time in his career.
Nigerians will be hoping the former FIFA U-20 highest goalscorer replicates his fine form for the Super Eagles.
United will face Premier League side West Ham in the fifth round of the FA Cup.
Nine of their first team players and five staff members tested positive for COVID-19 on Thursday.
